Blow molding is a well-known technique used for manufacturing hollow plastic protective boots for automobile components. However, such boots typically have a complex shape which can present difficulties to blow molding such an article.
Protective boots used on automobile components, such as a boot for a constant velocity joint (CVJ), perform several functions including protecting the joint from outside dirt and debris, as well as maintaining lubricant around the joint. Many protective boots, however, must attach to automotive components which do not have a cylindrical external configuration to which the boot may be clamped. For example, due to the bearing configuration of typical constant velocity joints, the shaft to which one end of a CVJ boot must clamp is lobed, rather than cylindrical. Thus, in order to perform its protective functions properly, a CVJ boot requires a seal or bushing element at one end to be clamped between the boot and the shaft. The bushing has a tri-lobed internal configuration.
In use, however, the addition of this extra element increases the ways in which the protective boot may fail. For example, if there is an incomplete seal between the bushing and the boot, lubricant may be allowed to escape from the joint or contaminants allowed to enter.
Accordingly, there is a need for a single piece boot and bushing which provides a suitable external clamping surface and an internal irregular surface which can be configured to meet the requirements of a mechanical structure in association with which the product is to be used. Producing such a product is very difficult using conventional blow molding techniques. Conventional blow molding techniques cannot be used to comprise a lobed internal surface on a portion having a cylindrical outer surface because thermoplastic material spreads out evenly under the effect of blowing pressure in typical blow molding.
A technique which overcomes this problem is disclosed in the applicant's co-pending U.S. Patent application Ser. No. 08/694,234, filed Aug. 8, 1996, now U.S. Pat. No. 5,900,205, which is incorporated herein by reference. That application discloses a combined blow molding and compression molding technique in which a parison is confined in a mold and, prior to blowing, a core is introduced into the mold to move thermoplastic material axially, radially and circumferentially to form an integral boot and lobed bushing product. The method disclosed in that application has the limitation, however, that only a certain amount of thermoplastic material may be moved by the introduction of a core or slides into a mold prior to blow molding, thus limiting the size of lobes which may be formed on a bushing portion of an integral bushing and boot.
Accordingly, there is a need for a process which is capable of molding an integral thermoplastic boot and bushing having unlimited lobe size or having other internal sealing surface which is non-cylindrical.